The World Health Organization reports that floods can increase the transmission of multiple infectious diseases, including water-borne infections like cholera and hepatitis A, and vector-borne diseases like dengue, West Nile, and malaria. The specific illnesses that will emerge during a flood vary based on a region’s ecology and epidemiology, but from Haiti to Houston, New Orleans to South Asia, significant flooding should increase public vigilance of infectious diseases. According to the WHO, though, the risk of infection is usually low “unless there is significant population displacement and/or water sources are compromised.” If water sources are compromised, prompt detection and action can still continue to mitigate risk.
During extreme flooding, sewage systems may spill out into roadways. Drinking water reservoirs can rupture and levees burst, causing clean water to mix with rainwater and sewage. The result? Contaminated floodwater that infiltrates everything, from property to people. Water-soaked buildings are a mold’s paradise. Hurricane season also coincides with mosquito season, and pools of standing water – clean or dirty – are breeding grounds for these disease vectors.
Besides ingestion, water-borne diseases can be contracted by skin contact with contaminated water; for instance, someone with an open wound is susceptible to developing an infection. Leptospirosis, for example, is a zoonotic bacterial disease of public health concern in some parts of the world. It is transmitted through the urine of infected animals, which can seep into water or soil and persist for months. Human contact either directly with urine or with contaminated water enables bacteria to enter the body through skin abrasions or mucous membranes (eyes, nose, and mouth). Leptospirosis causes a range of symptoms including fevers, chills, aches, jaundice, abdominal pain, and diarrhea. Severe, untreated infections can lead to kidney or liver failure, or meningitis.
In developing countries, flooding can lead to outbreaks of dangerous illnesses like cholera, typhoid, and yellow fever. In developed nations with robust public health infrastructure, flooding is more often accompanied by enteric diseases like E. coli, respiratory infections, and conjunctivitis. The WHO has stated that none of these pathogens – from cholera to conjunctivitis – are of extremely high epidemic risk after a natural disaster. More often than not, outbreaks keep to a small cluster of cases.
Of course, exceptions occur which later serve to inform future public health efforts. For example, after Hurricane Katrina, the CDC reported a greater than two-fold increase in West Nile Virus, which is transmitted by mosquitoes. This was not until after Katrina itself had passed, when affected regions in Louisiana and Mississippi contained large pools of standing floodwater (in contrast, during the actual hurricane, turbulent water would have the opposite effect on mosquito breeding grounds). Now, in the wake of Hurricane Harvey, people are speculating about the same public health issues; namely, whether certain mosquito-borne illnesses like West Nile Virus (which is endemic in Texas) or even Zika might increase in transmission in waterlogged areas. This, again, would likely be a delayed consequence rather than a direct one, since shortly after a hurricane water is still flowing and interrupting any existing insect breeding sites.
Altogether, the risk of major outbreaks is low in the wake of flooding. Public health-focused regulations such as boiling orders for water consumption help further push existing risks down. There are, of course, exceptions – such as the devastating cholera epidemic that struck Haiti shortly after a massive earthquake decimated much of the country. The source for this outbreak, however, was not the natural disaster itself. Instead, cholera was imported by UN aid workers from Nepal, who excreted the bacteria into poorly-contained latrines.
As one study outlines, one of the largest risk factors for outbreak after disasters is associated with population displacement. The availability of safe water, the underlying health status of a population, the presence of proper sanitation facilities, the degree of crowding, and the availability of healthcare services are further all interconnected in a complex ecological web that dictates the risk of an epidemic anywhere. Nonetheless, floodwater, especially in the event of sewage overflow and levee rupturing, is considered unsafe to consume. Individuals with skin abrasions such as cuts or dermatitis are advised to take proper precautions against coming into direct contact with potentially contaminated water, as is everybody else when it comes to drinking tap water in the wake of a natural disaster.
Kayla Knilans received her Ph.D. from the University of North Carolina and currently studies inflammatory mediators and epithelial repair in inflammatory bowel disease.
Climate change discussions frequently focus on the geological and ecological effects of climate change, such as rising sea levels and changing or extreme weather patterns. A less frequently explored aspect of climate change in the media is its impact on infectious diseases. But this is a critical issue for consideration in climate change discussions, as climate and infectious diseases are strongly linked. Temperature, humidity, rainfall, and even sunlight exposure and wind can have an impact on communicable illnesses. These effects can be directly on the pathogen, on their vectors or hosts, or on their living environment. Weather, then, can impact both the timing and the severity of disease outbreaks. In this context, climate change is not only an issue of environmental preservation, but one that is also tightly linked to public health.
Based on historical data from El Niño, La Niña, droughts, floods, and major weather events such as hurricanes, scientists have built models to predict the infectious diseases most likely to become greater threats to public health as a consequence of climate change. Predicting the impact of climate change on some diseases can be difficult due to confounding interactions with the climate. For example, while influenza viral particles are most stable at low humidity (20%–40%), viral stability is minimal at intermediate humidity (50%), and then rises again at high at elevated humidity (60%–80%), before falling sharply at very high humidity ( > 80%). In other cases, there is more consensus on the impact of climate change on infectious diseases, such as with vector borne diseases.
Vector-borne diseases that are transmitted by mosquitos are of particular concern, as the habitable environment for two critical species of mosquitos, Aedes aegypti and Aedes albopictus, has been expanding. Consequently, mosquito-borne infections have been on the rise. The global distribution of malaria, for example, is expected to increase as a consequence of warmer global temperatures. One area of particular concern and significant study are the highland regions of Africa where malaria is currently not endemic, but has historically seen increases in disease in warmer years. Dengue virus is also expected to expand its global distribution, with both the United States and Japan having reported local infections after nearly a century of absence from those countries. In addition, the 2015 Zika virus outbreak that has become a major ongoing public health issue was found to be fueled by the combination of El Niño that year and overall warming temperatures.
Social and economic factors also play a significant role in assessing the risk for infectious diseases as it pertains to climate change. These factors include access to clean water, a public health system with good infrastructure and an ability to respond to outbreaks, the presence of biosurveillance programs to identify outbreaks, and accurate early-warning systems for major weather events. Organizations such as the United Nations Environment Programme, the World Health Organization, and the Intergovernmental Panel on Climate Change, seek to provide countries with specific policy recommendations to mitigate disease outbreaks. Organizations like the Collaborative Adaptation Research Initiative in Africa and Asia seek to identify regions most vulnerable to the effects of climate change and support research and policy initiatives in those regions. The University of Notre Dame’s Global Adaptation Index has found that while many such countries are still vulnerable and have a low readiness to respond to climate change, they are making significant strides in reducing their susceptibility. Kenya, Bangladesh, and Burkina Faso, for example, have all prepared action plans and established financing for combating climate change. These countries in particular will need robust scientific research to inform the link between climate change and infectious disease to ensure good policy decisions and the proper allocation of resources.
A recent review article identified a critical lack of collaboration between scientific groups attempting to predict the epidemiological impact of climate change on infection risk and basic science researchers that work to understand the direct impact of climate variables on pathogens. Moving forward, scientific collaborations will be critical for predicting the changing patterns of infectious diseases so that countries can be well informed of the resources that will be needed to combat and prevent future outbreaks. It is also essential that countries are willing to enact policies that will both slow the progression of climate change and allocate resources for outbreak containment and prevention, including resources for less developed nations.
Francesca Tomasi received her B.A. from the University of Chicago and currently does tuberculosis drug discovery research.
Whenever an infectious disease pops up somewhere, one of the first things epidemiologists want to know is how contagious the pathogen is. Understanding how quickly an illness may spread is essential in both deducing whether a public health intervention is warranted and coming up with a strategy. Mathematically, contagiousness is described by a number called R0, which is pronounced “R naught.” R0 tells you the average number of susceptible individuals who will contract an infection from one contagious person. Susceptible individuals are those who have not been infected by this agent and who have not been vaccinated against it.
There are three outcomes that stem from knowing a pathogen’s R0, each of which carries its own implications for whether an epidemic, a widespread occurrence of disease, will occur within a population.
For infectious diseases with an R0 greater than 1, appropriate measures need to be taken to curb a brewing outbreak. Suffice to say the larger the value of R0, the more difficult a pathogen is to control.
What goes into calculating a pathogen’s R0? The three main factors are infectious period, contact rate, and mode of transmission. The longer someone is contagious, the higher the chances that person will spread their illness to more people. Similarly, the more frequently a contagious person interacts with susceptible individuals, the more people are likely to get infected. This plays into visible versus invisible illness too: if someone is visibly ill, they are more likely to stay at home (or in a hospital) and avoid contact with the outside world. Furthermore, if somebody is diagnosed with an infectious disease, they may even be quarantined, a practice that dates back thousands of years, before people even knew what caused communicable diseases. The term “quarantine” itself comes from the Venetian dialect of the Italian phrase quaranta giorni, which means forty days. During the Black Death in the 14th century, ships were isolated for 40 days before their passengers could enter cities. Lastly, how a disease spreads will dictate how contagious it is. For instance, something that spreads through the air – like the flu – does not necessarily require physical contact and thus spreads quickly and easily. On the other hand, diseases like HIV that are transmitted by bodily fluids, are harder to catch and spread.
Let’s look at some common infectious diseases and their R0 values:
Just as with any tool, R0 carries its own limitations. In the event of vector-borne diseases like malaria, it is difficult to predict frequency of mosquito bites and boil it down to an average. Furthermore, R0 is often attributed as a threshold rather than an absolute quantity. Regardless, it is helpful to understand disease dynamics when informing public health interventions in the event of an outbreak.